Evidence of growing bred vector associated with the tropical intraseasonal oscillation

ments for the tropical ISO, we have to examine whether the specified initial perturbations are adequate for the tropical atmospheric motions. Kubota et al. [2005] indicated that initial perturbations produced by a Breeding of Growing Modes (BGM) method [Toth and Kalnay, 1993, 1997] in the operational ensemble prediction system of the Japan Meteorological Agency (JMA) are characterized by extratropical baroclinic unstable modes. In addition, kinetic energy of these initial perturbations in the tropical regions is too large for assessing the predictability of the tropical atmospheric circulation. This is because the norm of the initial perturbations in the operational BGM method of the JMA system is defined by the 500-hPa geopotential height variance. Thus, we modify the BGM method to create initial perturbations that are adequate to evaluate the predictability of the tropical ISO. [4] In this paper, as a first step to evaluate the predictability of the tropical ISO, we will examine dynamical properties of initial perturbations obtained by a modified BGM method using the operational ensemble prediction system of the JMA. We modify several parts of the operational BGM method to create perturbations suitable for the tropical tropospheric circulation. As a result of this modification of the BGM method, growing perturbations associated with the tropical ISO are detected in the tropical regions, which suggests that the tropical ISO is linearly unstable to infinitesimal perturbations.